Electro-optic modulators are devices that impress the information in an electrical signal onto an optical carrier. They are used for commercial fiber-optic telecommunications, photonic signal processing, and military electro-optic applications. Commercial modulators used in telecommunications have a typical bandwidth of 25 GHz, which enables data rates of about 100 Gb/s using quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM) modulation formats. The highest bandwidth available commercially is 40 GHz. Future-generation telecommunications systems have increasingly higher data-rate requirements (e.g., 200 Gb/s, 300 Gb/s, etc.) that cannot be satisfied by the current state-of-the-art modulators.
There have only been sporadic reports of research modulators that operate at bandwidths above 40 GHz. The Prather group at the University of Delaware has reported a research-grade phase modulator made in LiNbO3 having a 3-dB bandwidth of 94 GHz, but the half-wave drive voltage (Vπ) and optical insertion loss (OIL) are high at 7 V and 3.7 dB, respectively, and this modulator is not available commercially. In addition, LiNbO3 suffers from many deleterious effects including the photorefractive effect, the photovoltaic effect, and charge migration which can dynamically alter the performance of the devices. Gigoptix, Inc. has reported sample quantities of polymer-based modulators having a bandwidth of 70 GHz, a half-wave voltage Vπ of 6.8 V, and an OIL of 7 dB, but this modulator suffers additionally from a low power-handling capability due to polymer bleaching and lifetime effects (<10 mW estimated). Further, the packaged polymer modulator unit requires a DC bias of 70-180 mA to operate within specifications.
Compound semiconductor modulators made in GaAs- or InP-based materials offer improvements to these parameters but so far the performance demonstrated by developers has fallen far short of the bandwidth requirement of 100 GHz. The most significant body of work in the U.S. on this topic is from the Dagli group at University of California at Santa Barbara, whose best published measurements on a Mach-Zehnder modulator show a bandwidth of 39 GHz, a Vpi of 8.4 V (1 mm device length), and an intrinsic OIL excluding input/output coupling losses of 2.9 dB. The optical waveguide structure used in this device was a GaAs core clad by AlGaAs layers, and the device was fabricated using substrate removal and subsequent bonding onto a semi-insulating (S.I.) GaAs carrier wafer. The microwave electrode structure in this device is a coplanar waveguide that is capacitively loaded with periodic T-rails to slow the microwave velocity to match the optical velocity.
Capacitive T-rail loading of the microwave line has been a standard approach for velocity matching in modulator design since the 1980's, but it has a number of drawbacks. First, the fill factor F (ratio of T-rail length to T-rail unit periodicity) is generally undesirably low (e.g., about 0.5) to maintain a characteristic impedance Z0 of 50Ω; raising F to 0.9 or better reduces the impedance to undesirable levels typically between 15Ω and 35Ω. Maintaining the impedance at 50Ω also directly hampers OIL by about 3 dB as the optical waveguide must be made longer by a factor of 1/F. Second, the use of T-rails that are optimized for optical modulation and transmission-line velocity is projected to give an additional 6 dB/cm of microwave loss at 100 GHz over an unloaded transmission line alone, which is unacceptable for many applications. (For perspective, an electrical loss of 0.6 dB/mm would yield an E/O bandwidth of about 100 GHz for a 1-cm-long device.)
InP-based modulators on Si substrates have been demonstrated with bandwidths as high as 27 GHz, Vpi values of 4.8 V, and extrinsic OIL values of 4.5 dB for modulator lengths of 500 μm. However, the propagation loss in the microwave electrodes in their later design was measured to be 9 dB at 40 GHz and is projected to be over 16 dB at 100 GHz, which is excessive. Without being bound by any particular theory, this loss may arise from two components: carrier-related loss in the InP quantum wells and doped layers, and loss due to field penetration into the Si substrate.
An electro-optic modulator design with sandwich electrodes containing a high-permittivity material is claimed to be usable at 100 GHz, but there may be some potential issues with the design and more rigorous modeling of the design needs to be done. Another electro-optic modulator design uses integrated active drive employing InP-based HEMTs in a distributed-amplifier arrangement. The possibility of operation up to 15 GHz is shown for this design. Additionally, this design has greater fabrication complexity and requires that three velocities be matched: the gate-line velocity, the drain-line velocity, and the optical velocity.